California Code, Vehicle Code - VEH § 24617

(a) A transit bus may be authorized to be equipped with a yield right-of-way sign on the left rear of the bus.  The yield right-of-way sign may flash simultaneously with the rear turn signal lamps, but is not required to do so.  The sign may also be a static decal that is affixed to the left rear of the bus.  If the sign is a flashing light-emitting diode (LED) sign, it shall be both of the following:

(b) This section does not require a transit agency to install the yield right-of-way sign described in subdivision (a).

(c) This section does not relieve the driver of a transit bus from the duty to drive the bus with due regard for the safety of all persons and property.  This section does not exempt the driver of a transit bus from Section 21804.

(d) This section applies only if the governing board of the transit agency approves a resolution, after a public hearing on the issue, requesting that this section be made applicable to it.

(e) A participating transit agency shall undertake a public education program to encourage motorists to yield to a transit bus when the sign specified in subdivision (a) is activated.